In recent years, there have been known routing assistance devices that acquire map information from an information center via a communication network and gives route guidance using the acquired map information. With such a routing assistance device, the latest map information in the possession of an information center is always available without changing a map disk such as DVD-ROM when map information is updated.
However, there is limitation on the amount of map information acquirable from an information center at a time because of the limited size of the storage area in a routing assistance device, the limited channel capacity of communication networks, a limited communication time, and the like. To cope with this, the technique described in Patent Document 1 is constructed so as to perform the following operation: when there is an area that contains a route to a destination and is not possessed as map data by a routing assistance device, map data on this area is acquired from an information center. That is, the amount of information acquired from an information center is reduced by acquiring only map information on an area used in route guidance of map information from among multiple divided areas.    Patent Document 1: JP-A-2002-340573
However, the invention disclosed in Patent Document 1 may pose a problem, e.g., when a new road, which is connected with a guided route leading to a destination and which is also able to reach the same destination, is opened to traffic. In fact, all the necessary road map data covering this new road to the destination may not be entirely downloaded from the information center to the routing assistance device. Thus, a user may not learn where the new road connects. As a result, it is difficult for him/her to think of and select an alternative route getting off the current guided route and going through the new road at his/her own will. That is, it is not easy for the user to deviate from the guided route and select the road newly opened to traffic at his/her own will.
It is expected that the storage area of routing assistance devices and the channel capacity of communication networks will be expanded thanks to technological advances. Thus, it will be possible to increase the amount of information acquired from an information center as compared with the present situation.